Field of the Invention
This invention relates to a novel catalyst, suitable for oxychlorination of hydrocarbons. It is especially concerned with a catalyst for the oxychlorination of ethylene to 1,2-dichloroethane (EDC).
The vapour-phase oxychlorination of ethylene to EDC using a fixed bed reactor containing a supported catalyst, usually a supported copper catalyst, is widely used commercially, for example as part of the process for producing vinyl chloride monomer (VCM). The industry is constantly seeking to improve the efficiency of the process, and much work has been reported on the effects which different catalysts have on the process. Thus both the composition and physical presentation of the catalyst has been studied. The present invention is concerned in particular with the physical shape of the catalyst.
Over the last few years improvements have been reported in catalytic performance obtained by suitable modification of the shape and/or size of catalysts in pellet form. Such characteristics affect some of the most important properties of the catalytic bed in fixed bed reactors, such as i) the resistance of the reactant flux (pressure drop), which determines the maximum possible flow through the reactor; ii) the efficiency of heat exchange, which allows the removal of heat from the highly exothermic oxychlorination reaction; and iii) the effectiveness of the pellet as far as the diffusion of reactants and reaction products inside the pellets is concerned.
A low pressure drop allows the flow through the catalytic bed, and therefore the productivity of industrial reactors, to be increased. On the other hand because a reason for replacing the catalysts in industrial reactors is the increase of the pressure drop with the catalyst life, an initial low pressure drop allows a larger range of pressure drop and consequently a longer use of the catalysts before they need replacing. Starting from the usual catalysts, shaped as spheres or solid cylinders, a lower pressure drop through the catalytic bed has been obtained by developing catalysts based on columnar configurations, through hollowed pellets shaped with circular or multilobed cross-sections, which give rise to catalytic beds with higher void fractions.
Catalysts of this type, for use in oxychlorination reactions, have been described for example in the following patent specifications. U.S. Pat. No. 4,366,093 describes a hollowed cylindrical catalyst having an outer diameter De in the range 3-6 mm, an internal diameter Dixe2x89xa71 mm, a wall thickness of at most 1.5 mm and a length L in the range 3-6 mm.
U.S. Pat. No. 4,382,021 and EP-A-054674 report a hollowed cylinder catalyst having the dimensions De=5-12 mm, Di=3-8 mm and L=3-12 mm.
U.S. Pat. No. 4,740,644 claims a new method for preparing hollowed cylinder catalysts, and exemplifies catalysts with De=5 mm, Di=1.8 mm and L=5 mm.
In U.S. Pat. No. 5,166,120 a catalyst prepared via extrusion, shaped as a hollowed cylinder with De=4-6 mm, Di=1-2 mm and L=1.7-3.75 De is described.
WO 96/40431 describes a catalyst for ethylene oxychlorination which is shaped as a hollowed cylinder with internal reinforcing vanes, with Dexe2x89xa76.5 mm, wall thickness in the range of 0.1-0.3. De and L=0.5-5. De.
Hollow cylindrical pellets have a S/V (geometric surface to volume ratio) higher than spheres and solid cylinders, and this, together with a higher catalytic bed voidage, gives a more efficient heat exchange. Thus, better temperature control along the catalytic bed and reduced hot spot temperatures are obtained: in this way a longer catalyst life is achieved and the reaction results in a reduced formation of chlorinated by-products and combustion products.
A further benefit of hollowed cylindrical pellets, due to the higher geometric surface combined with a lower wall thickness of the pellets, is the higher effectiveness of the pellets, because the reaction takes place only in a thin external layer. Moreover, the formation of carbonaceous deposits inside the core of the pellet wall, which causes pellet breakage and pressure drop increase during industrial run, is reduced. Consequently, a further increase of productivity and catalyst life can be obtained.
In spite of the above described advantages a hollowed pellet must be designed carefully, since otherwise several disadvantages become evident. For example, if the Di/De ratio of a hollowed cylinder is greater than a certain value, the pellet becomes too fragile, without further advantage in terms of effectiveness. Moreover the apparent bulk density of the catalyst decreases, resulting in a lower conversion per unit volume of catalyst bed due to the lower total active phase content. This last effect can affect also the catalyst life, because the catalyst tends to lose active phase compounds in the reaction environment. A solution to this problem is to increase the active phase concentration of fresh catalyst, also because an excess of active phase compounds, even if not contributing directly to the catalyst activity, can act as a reservoir for the pellet, increasing the catalyst life. However the Cu concentration can not be increased over a certain extent, because the consequent loss of the catalyst surface area causes a loss of activity.
The above described problems can also be encountered if a more favourable pressure drop of catalyst shaped as hollowed cylinders is pursued by increasing De or L with a constant Di/De ratio. A further disadvantage of this approach is that a too high increase of De or L can cause an inhomogeneous loading of the catalyst inside the reactor.
The above remarks make it clear that, in terms of oxy catalysts in pellet shape it must be taken into account that every change capable of giving rise to some improvement in catalytic performance can also cause unwanted detrimental effects, especially if the changes are not balanced carefully by the simultaneous modification of other characteristics. As a conclusion, in order to obtain an excellent oxy catalyst it is not sufficient to optimise a single characteristic; all the properties as a whole, responsible for different effects, must be carefully balanced.
The present invention has for its primary object to provide a catalyst for effective use in oxychlorination reactions. A further object of the invention is to provide a catalyst which satisfies the above described requirements of lower pressure drop of the catalytic bed, better heat exchange and good effectiveness without the disadvantages reported above.
According to the invention there is provided a catalyst comprising a carrier and catalytically-active material comprising copper supported thereon, the copper being present in an amount of 1-12 wt % on the dry catalyst, wherein the catalyst is in the form of a hollow cylinder having the following dimensions:                     4.0        ≤                  D          e                ≤        7.0                            (        1        )                                2.0        ≤                  D          i                ≤        2.8                            (        2        )                                6.1        ≤        L        ⁢                  xe2x80x83                ≤        6.9                            (        3        )                                          xe2x80x83                ⁢                  2.0          ≤                                    D              e                        /                          D              i                                ≤          2.5                                    (        4        )            
wherein De is the external diameter (mm), Di is the internal diameter (mm) and L is the length (mm), respectively, of the hollow cylinder.
The invention also provides the use of such a catalyst in the oxychlorination of hydrocarbons, especially the vapour phase oxychlorination of ethylene to EDC.
In the preferred form of the catalyst of this invention, the hollow cylindrical pellets have dimensions De=4.5 to 5.5 mms, Di=2.0 to 2.6 mms, and L=6.2 to 6.6 mms and De/Di is in the range 2.1 to 2.3. The catalysts of the invention are especially effective when used in tubular reactors having diameters in the range 25 to 50 mms.
The carrier material of the catalyst of the invention may be any of the materials known for producing copper-supported catalysts. Examples include silica, pumice, diatomaceous earth, alumina, and other aluminium hydroxo compounds such as boehmite and bayerite. The preferred carrier materials are xcex3-alumina and boehmite, the latter normally being pre-heat treated to convert it into alumina. The carrier material suitably has a surface area (BET) of 50-350 m2/g.
The catalytically-active material supported on the carrier contains copper in an amount of 1-12 wt %, based on the weight of the dry catalyst. The copper will normally be deposited on the carrier in the form of a salt, especially a halide, and preferably as cupric chloride.
The copper may be used in combination with other metal ions, in order to assist in the attainment of the desired selectivity and conversion performance. Such other metals include, for example, alkali metals (such as Li, Na, K, Ru, Cs), alkaline-earth metals (such as Mg, Ca, Ba), group IIB metals (such as Zn and Cd) and lanthanides (such as La, Ce and so on) or a suitable combination of them. These additional metal ions can be added as salts or oxides, the total amount of additives suitably being in the range 0-10 wt %. They can be added together with the copper or alternatively one or more of them (even all) after or even before the copper. In the last case their addition can be followed by an intermediate heat treatment. Preferred alkali metals are Li and K and they are preferably added as chlorides, each of them in the range 0-6 wt %. The preferred alkaline-earth metal is Mg, added in the range 0-6 wt %. Preferred lanthanides are La and Ce, each of them added in the range 0-6 wt %.
The addition of the catalytically-active components can be accomplished by methods well known by those of skill in catalyst preparation. There may be mentioned, for example, dry impregnation, incipient wetness impregnation or dipping, using a suitable solution of compounds to be added, for example an aqueous solution, optionally containing also acids such as HCl.
The addition of the active components can be made partially or totally before or after the formation of the hollow pellets. Preferably the catalysts are prepared by impregnation of the already formed carrier.
The shaping of the carrier or the catalyst may be performed by well known methods such as tabletting and extrusion. These operations are performed in the usual manner, optionally using additives such as lubricants and/or binders. Preferably the shaped pellets are obtained by tabletting, to attain a more uniform pellet size, density and higher mechanical resistance. The operations include customary thermal treatments, such as calcination of the carrier at 500-1100xc2x0 K, preferably at 750-950xc2x0 K, if the active part is added to the carrier after the shaping procedure and drying at 330-500xc2x0 K after addition of the active components.